1. Field of the Invention
The present invention relates to a condenser microphone including a variable directivity function by including two microphone units back to back, and especially relates to a variable directivity electret condenser microphone that uses electrets for the condenser microphone units, and outputs an audio signal in a balanced manner.
2. Description of the Related Art
As a microphone that can vary directivity, one in which microphone units having a cardioid characteristic are arranged in front and back in a back to back manner has been proposed.
Typically, condenser microphones are suitable for collection of wideband sounds compared with dynamic microphones, and are superior in directional frequency response.
One that realizes the variable directivity by adding/subtracting a polarization voltage to be added to the respective condenser microphone units by taking advantage of characteristics of the condenser microphones is disclosed in “Condenser microphone with variable polar response”, Microphone Engineering Handbook (p. 32, FIG. 1.18) written by Michael Gayford (Non-Patent Document 1).
Meanwhile, the applicant of the present application has an earlier filed patent application about a variable directivity condenser microphone that has overcome technical problems in the condenser microphone disclosed in Non-Patent Document 1, and this patent application is disclosed in JP 2012-65147 A.
According to the condenser microphone disclosed in JP 2012-65147 A, a decrease in output sensitivity and deterioration of S/N caused due to alternating current coupling of the front and back diaphragms like the condenser microphone disclosed in Non-Patent Document 1 can be prevented.
In the cases of using the two condenser microphone units that require the polarization voltage disclosed in Non-Patent Document 1 and JP 2012-65147 A, it is necessary to include a configuration that the polarization voltage of 60 V or more is obtained by a DC-DC converter or the like using a direct-current power source of about 5 to 20 V, which operates a circuit of an impedance converter or the like.
According to the above configuration, it is also necessary to include an auxiliary configuration of the above-described DC-DC converter and the like in the condenser microphone units, and thus it is inevitable to have an increase in the cost.
Therefore, the applicant of the present application also has a patent application about a variable directivity condenser microphone using an electret dielectric film in the two condenser microphone units, and this application is disclosed in JP 2008-118260 A. According to the variable directivity condenser microphone disclosed in JP 2008-118260 A, outputs of the two condenser microphone units are coupled with a variable-capacity capacitor (variable capacitor), whereby a microphone that can continuously change the directivity can be realized.
By the way, JP 2012-65147 A also discloses an example of a variable directivity condenser microphone using an electret condenser microphone unit that does not need the polarization voltage. According to an example using the electret condenser microphone unit disclosed in JP 2012-65147 A, phase adjustment means is employed, which includes a phase inverting amplifier in which a phase of input/output is inverted with a gain of “1”, and selects outputs of the phase inverting amplifier and a non-inverting amplifier in which the phase of input/output is not inverted with the gain of “1”. Therefore, employment of the phase adjustment means has a problem of complexity of a circuit configuration, and thus there is room for improvement.
Further, according to the variable directivity condenser microphone disclosed in JP 2008-118260 A, the configuration of coupling the outputs of the two electret condenser microphone units with the variable-capacity capacitor (variable capacitor) is employed. Therefore, an electrostatic capacity is changed when external vibration is added to the variable-capacity capacitor, and this becomes a cause of occurrence of noise.
Therefore, a measure against the external vibration that affects the variable-capacity capacitor is required, and there is room for improvement on this point.
Meanwhile, in this sort of microphones, when a considerable length of microphone cord is required in a case where the microphones are used in a hall, an event site, or the like, or when the microphones are used for professional use such as to authentically collect sounds of music with a high S/N in a studio or the like, a balanced shield cable is used as the microphone cord.
Therefore, a balanced output system is employed for a sound output of the microphones used in the above cases, audio signals from the microphones are sent to a mixer and the like through the balanced shield cable. Then, the mixer side extracts the audio signals using a differential amplifier or a microphone transformer, thereby to obtain audio signals with reduced common mode noise.